1. Field of the Invention
The present invention relates to ultrasonic spot bonding. More particularly, the present invention relates to a new and improved edge bonding apparatus for making rolling spot bonds between conductive fingers and pads on a substrate of a semiconductor or solar cell.
2. Description of the Prior Art
The pads and electrodes on semiconductor chips are usually wire bonded to make conductive interconnections between the pads on the chip and the conductive lead-out patterns which connect to the pins inside a semiconductor package such as a dual in-line package or DIP. The pads and electrodes are so close together on tightly packed integrated circuits that the wires also provide a means for spreading or fanning out the conductors so that the connection pins on the packages of the integrated circuits are reasonably spaced.
It is known that very small etched foil patterns comprising a plurality of conductive fingers can be individually ultrasonically bonded or thermocompression gang bonded to pads and electrodes on semiconductor chips if specially prepared bumps are plated on either the pads or the conductive fingers. Since the foil pattern or fingers are usually made on a dielectric supporting tape this technique and variations of this technique are known as tape automated bonding or TAB. The bonding tool employed to make bonds between the foil fingers and the special pads or bumps may be shaped to bond all of the fingers simultaneously or to bond one or more of the fingers at one time. The bonding tool is referred to as a wedge or tab bonding tool and is not usually employed to mash the fingers being bonded in the same manner that a wire bond is mashed when a wire is bonded at the second or last bond.
Heretofore, ribbons of thin flat strips of conductive metal have been employed in the semiconductor industry to make interconnections. When such ribbons are ultrasonically bonded to a semiconductor device or device substrate, the ribbon is mashed by the wedge bonding tool to make a proper molecular interconnection between a terminal or pad and the ribbon. Such ribbons have been bonded by vertically moving wedges and by horizontally rolling wheel shaped ultrasonic transducers.
When the manufacturers of solar cells were faced with the problem of making solar cells economically, all of the abovementioned methods of making thermocompression and ultrasonic bond interconnections were considered as well as solder reflow techniques. While solder reflow and roller seam welding were deemed to produce feasible bonds in the laboratory they were either too costly or did not produce commercially usable products. Solder reflow is slower, requires flux and special cleaning before and after bonding as well as expensively prepared foil for making interconnections.
Roller seam welding entails the use of large powerful generators and transducers which cannot easily be tuned to optimum values. In addition to being slow and expensive the large mass of the roller seam welder does not lend itself to making consistent spot or seam bonds on small conductive fingers. Since solar cells are made on thin silicon substrates and are very brittle, the high bonding forces which are required to make roller seam bonds tends to crack the substrate and/or crash onto the cell material of the solar cell on either side of the finger which can crack the substrate or damage the substrate material. A solar cell may have eight to twelve dendritic web patterns and associated pads for collecting electrons. Each of the patterns will require that a finger be bonded on the front pads and the back of the web pattern. Up to 180 cells are employed in one panel or module, thus, about 3000 bonds per panel or module are usually employed in a commercially usable 60 watt module. If any two bonds are bad on any one cell or if the cell is broken and unusable the module must be repaired before it can be accepted. It would be desirable to provide an apparatus and a method for bonding thin strips, fingers or ribbons to the front and/or back of solar cells so that they may be connected in series and parallel arrays in modules that are to be manufactured economically without having to rework and repair the solar cells.